(1) Field of the Invention
The present invention relates to a semiconductor device and more particularly to an output circuit thereof.
(2) Description of the Prior Art
In a semiconductor dynamic memory, various clocks having predetermined timing relationships are used to activate respective parts of the memory. Furthermore, the power source voltage has been reduced from 12 V to a level of 5 V and, thus, the element size is reduced. Accordingly, the accumulated charge in the memory cell is very small. In a device where the size and the voltage of the power source are thus reduced, it is desired to increase the quantity of the charge stored in cells and cause the cells to operate sufficiently and stably by increasing the clock amplitudes beyond the power source voltage. For attaining this purpose, a bootstrap circuit and a charge-pumping circuit have been used.
However, an output-level guaranteeing circuit utilizing a charge-pumping circuit has a problem at the time of application of power. A clock pulse of the charge-pumping circuit is usually output from a substrate bias-voltage generating circuit, and the substrate bias-voltage generating circuit starts operating substantially simultaneously with the application of the power source voltage so that the clock pulse is simultaneously produced. However, clock pulses for activating a buffer circuit connected before the output circuit, are output from predetermined stages of the various clock pulse generators connected in multiple stages. Therefore, the predetermined logic levels of the clock pulse are not attained at the time of application of the power source voltage. When these clock pulses for activating the buffer circuit are all at a low level, the output is in the floating state.
When the clock pulse of the charge-pumping circuit becomes a high level with the rising of the power source voltage, the output voltage level rises for a reason which explained in detail later. If the output voltage level rises in such a manner, sometimes an abnormal current flows, and short circuiting of the power source may occur.